Abstract

Terahertz plasmonic resonances in semiconductor (indium antimonide, InSb) dimer antennas are investigated theoretically. The antennas are formed by two rods separated by a small gap. We demonstrate that, with an appropriate choice of the shape and dimension of the semiconductor antennas, it is possible to obtain large electromagnetic field enhancement inside the gap. Unlike metallic antennas, the enhancement around the semiconductor plasmonics antenna can be easily adjusted by varying the concentration of free carriers, which can be achieved by optical or thermal excitation of carriers or electrical carrier injection. Such active plasmonic antennas are interesting structures for THz applications such as modulators and sensors.

Figures (8)

Schematic representation of the scattering geometry. A monochromatic, linearly p-polarized incident beam impinges at an angle Ө on scatterers with frequency-dependent homogeneous permittivity ε. The distance between the scatters is Δ. r represents a generic point in the (x, z) plane and (R)(s) is a generic point on the parametric curve that delimitates each scatter.

Complex permittivity of InSb (a) at room temperature and of gold (b) at THz frequencies calculated using the Drude model. The black squares correspond to the real component of the permittivity (absolute value in (b)), while the red circles are the imaginary component.

Scattering efficiency of rectangular dimers (see Fig. 1) with different lengths L, illuminated with THz radiation polarized along L, the permittiviy of the background is one. The height h of the dimers is 5 µm and the gap Δ is 2 µm. The lengths are indicated in the figure legends. Figure (a) corresponds to calculations of InSb dimer antennas, while the calculation of (b) corresponds to gold antennas.

Near-field intensity distribution in logarithmic scale at the main resonance. (a) InSb (N=2⋅1016 cm−3) antenna with a gap Δ=2 µm, length L=50 µm, height h=5 µm and ν=1.62 THz. (b) Au antenna with same dimension as (a) but at ν=1.73 THz. (c) Horizontal cut of (a) at z=0. (d) Horizontal cut of (b) at z=0. (e) Zoom around the gap of (a). (f) Closed view of the gap of (b). The amplitude of the incident plane wave is one and impinges from the top at θ=0° (see of Fig. 1), the permittivity of the background is one.

Near-field intensity enhancement at the central point of the gap of a rectangular dimer InSb antenna with an electron concentration N=2⋅1016cm−3 as a function of the antenna length L and gap Δ. The height of the antenna is 5 µm. The electric field (p-polarized plane wave) impinges on the top at ν = 1.5 THz. The permittivity of the background is one.

(a) Near-field intensity distribution in logarithmic scale at the plasmonic resonance (ν = 2.1 THz) of an InSb bowtie antenna, the permittivity of the background is one. Each triangle of the bowtie has the base and height with a length of 20 µm and the gap is Δ=1 µm. The incident field, with an amplitude equal to one and a frequency of 2.1 THz, impinges from the top as illustrated by the white arrow. The perpendicular double arrow indicates the polarization. (b) Near-field intensity at z=0.

(a) Near-field intensity enhancement at middle position of the gap, of an InSb dimer antenna as a function of the electron concentration. The antenna dimensions are h=5 µm, and L=40 µm with different gap widths dimensions Δ. A plane wave of frequency ν = 1.5 THz and polarized along L impinges on the top of the antenna (θ=0°).,The permittivity of the background is one. (b) Minus the real component (blue triangles) and the imaginary component of the complex permittivity of InSb at ν = 1.5 THz as a function of the electron concentration.

Scattering efficiency of an InSb dimer antenna for various electron concentrations, The dimer has a gap Δ=1 µm, a length L=40 µm and a height h=5 µm, and it is illuminated from the top (θ=0°) with a plane wave polarized along L. The permittivity of the background is one.